The quantification of chemical and biochemical components in colored aqueous fluids, in particular colored biological fluids such as whole blood and urine and biological fluid derivatives such as blood serum and blood plasma, is of ever-increasing importance. Important applications exist in medical diagnosis and treatment and in the quantification of exposure to therapeutic drugs, intoxicants, hazardous chemicals and the like. In some instances, the amounts of materials being determined are either so minuscule--in the range of a microgram or less per deciliter--or so difficult to precisely determine that the apparatus employed is complicated and useful only to skilled laboratory personnel. In this case the results are generally not available for some hours or days after sampling. In other instances, there is often an emphasis on the ability of lay operators to perform the test routinely, quickly and reproducibly outside a laboratory setting with rapid or immediate information display.
One common medical test is the measurement of blood glucose levels by diabetics. Current teaching counsels diabetic patients to measure their blood glucose level from two to seven times a day depending on the nature and severity of their individual cases. Based on the observed pattern in the measured glucose levels, the patient and physician together make adjustments in diet, exercise and insulin intake to better manage the disease. Clearly, this information should be available to the patient immediately.
Currently a method widely used in the United States employs a test article of the type described in U.S. Pat. No. 3,298,789 issued Jan. 17, 1967 to Mast. In this method a sample of fresh, whole blood (typically 20-40 .mu.l) is placed on an ethylcellulose-coated reagent pad containing an enzyme system having glucose oxidase and peroxidase activity. The enzyme system reacts with glucose and releases hydrogen peroxide. The pad also contains an indicator which reacts with the hydrogen peroxide in the presence of peroxidase to give a color proportional in intensity to the sample's glucose level.
Another popular blood glucose test method employs similar chemistry but uses, in place of the ethylcellulose-coated pad, a water-resistant film through which the enzymes and indicator are dispersed. This type of system is disclosed in U.S. Pat. No. 3,630,957 issued Dec. 28, 1971 to Rey et al.
In both cases the sample is allowed to remain in contact with the reagent pad for a specified time (typically one minute). Then, in the first case, the blood sample is washed off with a stream of water while in the second case, it is wiped off the film. The reagent pad or film is then blotted dry and evaluated. The evaluation of the analyte concentration is made either by comparing color generated with a color chart or by placing the pad or film in a diffuse reflectance instrument to read a color intensity value.
While the above methods have been used in glucose monitoring for years, they do have certain limitations. The sample size required is rather large for a finger stick test and is difficult to achieve for some people whose capillary blood does not express readily.
In addition, these methods share a limitation with other simple lay-operator colorimetric determinations in that their result is based on an absolute color reading which is in turn related to the absolute extent of reaction between the sample and the test reagents. The fact that the sample must be washed, blotted or wiped off the reagent pad after the timed reaction interval requires that the user be ready at the end of the timed interval and wipe or apply a wash stream at the required time. The fact that the reaction is stopped by removing the sample leads to some uncertainty in the result, especially in the hands of the home user. Overwashing, overblotting or overwiping can give low results and underwashing can give high results.
Another problem that often exists in simple lay-operator determinations is the necessity for initiating a timing sequence when blood is applied to a reagent pad. A user will typically have pricked his or her finger to obtain a blood sample and will then be required to simultaneously apply the blood from the finger to a reagent pad while starting a timer with his or her other hand, thereby requiring the use of both hands simultaneously. This is particularly difficult since it is often necessary to ensure that the timer is started only when blood is applied to the reagent pad. All of the prior art methods require additional manipulations or additional circuitry to achieve this result. Accordingly, simplification of this aspect of reflectance reading instruments is desirable.
Great improvements have been achieved upon the introduction of the systems described in U.S. Pat. Nos. 5,179,005, 5,059,394, 5,049,487, and 4,935,346 wherein an apparatus is provided for accepting a test strip having a test pad, one surface of which comprises a reaction zone adapted to be optically readable by said apparatus. The test strip is inserted into the apparatus, the apparatus is started and then whole blood is applied onto the test pad. At least a portion of such blood is allowed to permeate to the reaction zone whereby any analyte present therein will react with color-producing reagents in the test pad to alter the light reflectance characteristics of the reaction zone. The reflectance of the reaction zone is then a measure of the presence and/or quantity of analyte present in the blood sample. As described in the aforementioned patents, this system does not require a large sample of blood nor does it require the user to undertake timed manipulations with respect to the beginning or end of the reaction. Instead, because the strip is first inserted into the apparatus prior to the application of the sample, a standard reflectance reading of the reaction zone in the dry state may be obtained. The beginning of the reaction can be detected by the first "breakthrough" of the liquid sample onto the reaction zone by monitoring the reflectance and comparing the reading to the standard reflectance of the dry reaction zone. A reflectance reading taken at a predetermined time after the reaction has begun when compared to the standard reflectance, i.e., the dry reaction zone reading, will be indicative of the quantity of analyte present in the sample.
While the above described system does indeed solve the problems of the prior art and relieves the user of the burden of measurement and timing, it does require that the user apply a sample of blood onto the strip while the strip is in the apparatus. For the most part this represents no problem to the vast majority of users. However, certain users suffer from handicaps such as poor vision or impaired motor coordination so that the accurate application of blood from such users' pricked fingers to the strip in place on the apparatus, represents a hardship. Further, for institutional users, for example, there is the possibility that some quantity of blood remains on the device from a prior user, since the systems necessitate applying one's pricked finger to the device. In such instances there is the need to disinfect the device between users.
Accordingly, for the above reasons, in the case of at least some users, it would be preferable to first apply the blood sample to the strip prior to inserting the strip into the apparatus. Unfortunately, by doing so the apparatus no longer has the capability of reading reflectance of the dry, unreacted, reaction zone, i.e., at no time is the dry reaction zone presented to the apparatus. This reading was necessary in the prior devices to provide a calibration standard for determining the reflectance change as a result of the reaction and hence the presence and/or quantity of the analyte in the sample.
In copending, commonly assigned U.S. Ser. No. 08/302,160, filed Sep. 8, 1994, entitled "Optically Readable Strip For Analyte Detection Having On-Strip Standard", incorporated herein by reference, there is described a strip, apparatus, and methodology for allowing the user to apply a sample to the strip before inserting it into the reading apparatus while also providing a calibrated standard. This above-referenced patent application teaches a strip which comprises a portion for having the liquid applied thereto, this portion having an optically visible surface (i.e., at least with respect to the optics of the apparatus to be employed with the strip) defining a reaction zone. The reaction zone is such that its reflectance varies as a function of the quantity of analyte present in the applied liquid. Preferably, such is accomplished by the analyte reacting with reactants to produce a color change of the reaction zone. The test strip further comprises a standard zone of contrasting reflectance, relative to the reflectance of the reaction zone. The standard zone is positioned on the strip so as to lead the reaction zone as the strip is inserted into the apparatus.
Accordingly, the apparatus may be provided with optical means for sequentially determining the reflectance value of the standard zone as the strip is being inserted into its fully inserted position in the apparatus and the reflectance value of the reaction zone after the strip has been inserted. Additionally, the apparatus is provided with means for calculating the presence and/or quantity of the analyte in question as a function of the standard zone reflectance and the reaction zone reflectance.
Owing to the configuration of the strip, and specifically, the provision of a standard zone leading the reaction zone, the aforementioned apparatus need be provided with only one set of optics, e.g., one light emitting diode and one light detector for reading the reflection at a single position along the path of the strip.
In operation, the user turns on the apparatus, applies the sample to a fresh strip and then inserts the strip fully into the apparatus and reads the results. Without intervention of the user, the strip, configured as described, allows the apparatus to read the reflectance of light incident upon standard zone as it passes the optics of the apparatus as the strip is inserted. This reading is then calibrated to account for variations owing to changes in the apparatus from the factory condition and to lot-to-lot variations in the strip. The fully inserted strip thereafter presents the reaction zone to the optics of the apparatus and the reflectance of this surface may be read. Means are provided for the apparatus to calculate and report the analyte presence or concentration as a function of these readings.
The above-described system has gone a long way toward easing the user's task in determining analyte concentration. It will be appreciated, however, that it is fundamental to the successful optical reading of a strip on which liquid has been applied, that the strip is inserted into the apparatus in a precise relationship with respect to the optics of the apparatus. It will be appreciated that when attempting to determine the presence of an analyte (and more importantly, when attempting to determine the quantity of an analyte) in a sample using optical techniques, consistent, precise presentation of the surface to be optically read must be adhered to. Absent such consistency, anomalous results will be reported by the highly sensitive instruments employed.
This need for a device insuring precision is difficult to satisfy in prior devices and hence the art has gone to substantial design efforts in attempting to provide it. Thus, for example, in U.S. Pat. No. 5,120,507, to Sano, a spring loaded plate is provided in a device for reading a strip. The spring loaded plate bears against the strip and holds it flat to the optical viewing means. Such a system aids in the precision of presenting the surface to be read to the optics, but falls short of solving the problem.
The precision problem is far more difficult to solve in the case of the system described in the aforementioned and referenced U.S. patent application Ser. No. 08/302,160 wherein not only is the reaction zone to be read by the optics but likewise, the leading portion of the strip is to be optically read and used as a calibrated standard zone for determining the analyte quantity. In such a system, rather than one area being read by the optics, a longitudinally extending leading portion of the strip is to be read as well. Moreover, this portion is to be read "on the fly" as the strip is being inserted. Further still, as described in the aforementioned patent application, the standard zone is essentially an area on a thin strip support which carries a relatively thick test pad having an exposed surface which, in turn, is to be read as the reaction zone. Accordingly, the strip varies in thickness along its longitudinal axis. Optical readings must be taken along this axis as the strip passes the optics during insertion and after having been fully inserted and, hence, the areas to be read must be precisely presented to the optics, the varying thickness notwithstanding. In the aforementioned patent application, a resilient ring like biasing means is described which has the property of providing biasing pressure against the strip when viewable to the optics of the apparatus irrespective of the varying thickness of the strip as it is being inserted. While such a method works, it is relatively expensive to incorporate into the device and, accordingly, alternatives have been sought.